Films of plastic material are currently available on the market which are suitable for being stretched very greatly, thereby reducing their thickness, and to some extent reducing their width. One of the applications of such films lies in wrapping, and the main purpose of stretching is to save film. Although such stretching imparts a high degree of plastic deformation, the film nevertheless retains some residual elasticity which is made use of when wrapping a product.
In all currently known apparatuses, film is stretched lengthways on a continuous basis by using at least two rolls, with the film being wrapped around each of them over a sufficient contact area for friction between the film and each roll to prevent slipping, and then by rotating the rolls in such a manner as to obtain different peripheral speeds. In this way, the speed of the film at the outlet from the two rolls is greater than its speed at the inlet, thereby generating a longitudinal force in that portion of the film which lies between the two cylindrical rolls, and thus stretching the film. It will be understood that the ratio between these two peripheral speeds is equal to the stretching ratio, and that the film may be elongated by up to 300% in this way (that is to say its initial length multiplied by 4).
Numerous ways of implementing these fundamental means are known. One of the simplest consists in driving the downstream roll by a motor and in braking the upstream roll in order to slow it down by a quantity corresponding to the desired elongation. A second implementation consists in coupling the two rolls mechanically and in stretching a film where it passes between them. It is also possible to assist the passage of the film by motorizing one of the rolls and by controlling the operation of the motor as a function of various parameters.
When it is desired that all of the stretching should be obtained by using two rolls only, problems arise fairly quickly due to the fact that the film is subject to a high degree of stress which often gives rise to breakages, in particular when the nature or the dimensions of the film vary or when the ambient atmosphere varies. In order to obtain satisfactory operation, it becomes necessary to reduce the elongation ratio, thereby reducing the risks of breakage and thus of stopping the apparatus.
One solution to this problem lies in using numerous stretching stages, with the film passing over a plurality of successive rolls whose peripheral speeds increase in the direction of film travel. One of the drawbacks of such a succession of rolls lies in the bulk of the resulting apparatus. A second drawback lies in the fact that the elongation ratio in such apparatus is fixed for each stage and any variation in the characteristics of the film or in the ambient atmosphere, or any change in the nature of the film can only be taken into account either by dismantling the coupling mechanisms or else by providing speed-varying means in the transmissions between the rolls, with such means being provided by devices which are specially shaped and expensive. Finally, mention should be made of more or less complex variants of the basic principle in which the elongation ratio is varied by replacing one of the rolls in the apparatus by another roll having a different peripheral speed, or by changing the diameter of one of the rolls in order to vary its peripheral speed for constant speed of rotation.
None of these prior devices can solve problems due to any kind of variation in the stretching conditions during a stretching run. One such variation which is of considerable importance lies in the thickness of the film within a given reel or between consecutive reels not being constant, due to the manufacturing process whereby the stretchable film is itself made. Another important factor is the operating temperature of the apparatus.
It has also been observed that the frequency of film breakage is much higher while the stretching process is being started up than it is in normal operation, and also that it is higher at lower ambient temperatures. In this context, it should be mentioned that stretching operations associated with wrapping are often performed in warehouses from which products are shipped under ambient temperature conditions which vary considerably from season to season.
The present invention seeks to provide a solution to this problem by means of apparatus which is simple and robust and which enables the stretching process to be regulated automatically as a function of variations in the conditions under which it is performed. This regulation is obtained without using servo-control means, i.e. without using any means for detecting various parameters and for processing signals representative of such parameters, which means are generally constituted by fragile members with a high risk of breakdown.